In the Garden:Western Mountains and High Plains
May, 2005
Regional Report

Lawn-thinning diseases are common during this time of year in the Rockies.

The Onset of Lawn Diseases

One of the most common lawn-thinning diseases in the Rockies is leaf spot/melting out. The disease is actually a complex of several fungi, that attack Kentucky bluegrasses, tall fescues, and rye grasses.

The leaf spot phase is most evident during cool, moist weather conditions in spring, early summer, and fall. Melting out is most active in warmer, dry conditions.

Visible Symptoms
The symptoms of leaf spot appear as small, brown-to-purple flecks on the grass leaf blades. Specks often become elliptical and may be surrounded by a dark purple border. Leaf tissue in the center of the spot may die and turn straw colored. If the spot enlarges across the leaf blade, the leaf withers and dies.

The melting out phase also starts as black-to-purple leaf spots, but as the disease progresses, the fungi work their way to the base of the plant, attacking the crown and roots. These basal plant tissues then turn dark brown and rot. This stage of the disease is termed "melting or fading out," since the grass begins to thin out.

Lawns will appear brown or straw colored, thin, and weak, with lots of irregular patches of dead grass. If you try to rake these areas, the dead grass will be easily pulled out.

The best way to prevent severe outbreaks of lawn diseases is good cultural management. This includes:
1. Plant disease-resistant varieties when planting a new lawn or renovating an established lawn.
2. Core aerate the lawn several times a year (spring, early summer, and autumn) to help reduce thatch buildup and improve soil conditions.
3. Mow the lawn with sharp mower blades that make even, clean cuts of the grass blades. Maintain a cutting height of 2-1/2 to 3 inches. Follow the 1/3 Green Thumb rule: Mow frequently enough to remove no more than one-third of the grass blade at each mowing.
4. Water deeply. Soak the lawn to a depth of 6 inches or more as infrequently as possible without inducing drought stress.
5. Avoid excessive applications of high-nitrogen fertilizers that make the lawn grow more rapidly with tender, succulent tissue that is more susceptible to the fungi. Apply nitrogen at the rate of one pound per 1,000 square feet of lawn four times a year. I like to time fertilizer applications around the holidays of Memorial Day, Fourth of July, Labor Day, and Halloween.

Fungicides (chemical controls) are rarely needed to control leaf spot diseases. Chemical control recommendations should be made by a qualified horticultural or turfgrass specialist and will be most effective when combined with good cultural management of the lawn.